Referring to FIG. 1, typical hourglass roller bearings 500, include an inner member 510, for example a ball, surrounded by an outer member 512. A plurality of hourglass shaped rollers 515 are disposed between the inner member 510 and the outer member 512. The inner member 510 defines a radially outwardly facing convex bearing surface 520. The outer member 512 defines a first radially inwardly facing convex bearing surface 522A and a second radially inwardly facing convex bearing surface 522B. Each of the rollers 515 has a concave exterior surface 525 extending between axial end faces 525A and 525B of the roller 515.
When bearings are operated at high loads, the typical rollers 515 can experience a detrimental effect referred to as “end-stress.” As shown in FIG. 2, each of the rollers 515 has an effective length LE that extends between axially outermost portions of two high stress zones 535A and 535B. The concave exterior surface 525 of the typical hourglass roller 515 has a constant radius of curvature RC over the entire effective length LE. As shown in FIG. 3, typically the peak surface stress 550P occurs in the high stress zones 535A and 535B. The peak surface stress 550P causes accelerated wear of: 1) the radially outwardly facing convex bearing surface 520; 2) the first radially inwardly facing convex bearing surface 522A; 3) the second radially inwardly facing convex bearing surface 522B; and 4) the concave exterior surface 525 of the roller 515.
For example, FIG. 4 illustrates the outer member 512 having a line of surface distress 555 extending circumferentially around a portion of the first radially inwardly facing convex bearing surface 522A, shown after testing (but before spalling) with a prior art hourglass shaped roller 515. FIGS. 5 and 6 illustrate the inner member 510 having lines of surface distress 555A, 555B, 555C and 555D at four locations where the high stress zones 535A and 535B of the prior art roller 515 contact the inner member 510, shown after testing (but before spalling). FIG. 7 illustrates additional wear in the form of spalling 566 on the radially outwardly facing convex bearing surface 520 of the inner member 510. FIG. 8 illustrates the prior art roller 515 having lines of burnished material 577 at an edge of the effective length LE that extends between the axially outermost portions of two high stress zones 535A and 535B.
Bearings are typically used in aircraft wing flap arrangements. For example, U.S. Pat. No. 8,714,493 describes a trailing edge flap arrangement for an aircraft wing that includes a drop linkage arrangement that includes one or more bearings. The subject matter of U.S. Pat. No. 8,714,493 is incorporated by reference herein, in its entirety.